Traditional systems for electrochemical energy storage and conversion include batteries, fuel cells, and electrochemical capacitors. Common features of these systems are that the energy-providing electrochemical processes take place at the boundary of the electrode/electrolyte interface and that the electron and ion transport from one electrode to another are separated in space.
Batteries are closed systems, with the anode and cathode being the charge-transfer medium taking an active role in the redox reaction, so that the voltage is generated on the electrodes, while a reducing or oxidizing reactants which provide chemical energy are stored in the same compartment. Fuel cells are open systems where the anode and cathode are just charge-transfer media and the active chemicals undergoing the redox 110 reaction are delivered from outside the cell. Usually these are oxygen from air, and fuels such as hydrogen and hydrocarbons from a tank. Energy storage (in the tank and air) and energy conversion (in the fuel cell) are thus separated in space.
In electrochemical capacitors (or supercapacitors), initially energy to charge the capacitor may be delivered both via redox reactions and/or with the external source of AC or even DC current. Redox processes result in the accumulation of energy-reach redox state of the material inside the capacitor. The capacitor is used for the charge separation between phases and finally generates an electric current during the discharge operation. In addition to this process, orientation of electrolyte ions at the interfaces leads to the initial charge and then discharge of electrical double layers in the same phase, which results in the energy-delivering movement of electrons in the external wire.
Both voltaic cells and fuel cells often have a special porous separator, so that pH and composition near cathodes and anodes can be kept different Ion-exchange membranes also may be used as a separator. Ions can easily and selectively penetrate through the membrane from one electrode to another inside the cell, while electrons are transported outside of the cell. Different redox potentials at the anode and cathode lead to the redox reactions at the interface electrolyte/electrode, charge separation and generation of the open circuit voltage between the anode and cathode. When the electrons move in a wire outside the battery from an anode to cathode, to keep total electroneutrality negative ions should move in the cell in the opposite direction through the membrane or it may be that positive ions, especially H+, are moving inside the cell through the membrane from the anode to the cathode. Positive and small H+ ions have the highest diffusion coefficient and can be present in high concentrations, leading to low internal electric resistance of the cell. This is why proton-selective ion-exchanging membranes are used in many galvanic cells.
The problem with fuel cells, including direct methanol fuel cells, is that oxygen reduction reaction, is very slow and a catalyst is needed. The electrodes in this case are usually made with expensive alloys or Pt, which lose their catalytic activity in the presence of different impurities, including CO2, CO and H2S.
The problem becomes even more complicated because exchange currents for anode and cathode reactions on Pt can be limited by both ion transport in solutions and interface electron transfer, thus leading to the current-overpotential, i.e. it is necessary to use voltage higher than its thermodynamic equilibrium value to start and conduct fast electrochemical reaction.
Application of fuel cells in underwater vehicles has another problem. Unlike ground and air transportation, these vehicles must carry both the fuel and the oxygen source because the oxygen concentration in water is insufficient to meet the vehicle power requirements. The oxygen source must possess a high oxygen content to accommodate the weight and volume constraints of the vehicle design, and to be amenable to safe handling and storage onboard submarines and surface ships. Gaseous oxygen storage does not provide adequate storage densities, while liquid oxygen storage introduces challenges with handling and storage. Liquid sources, such as hydrogen peroxide (H2O2), require compact, efficient, controllable conversion methods to produce oxygen and handle reaction byproducts. Solid-state oxygen sources such as sodium chlorate (NaClO3) and lithium perchlorate (LiClO4) possess high oxygen contents and are stable under ambient conditions; however, decomposition of these materials to gaseous oxygen typically employs thermal methods that are difficult to start, to stop, and to control. All this clearly demonstrates the necessity to find innovative approaches to use chemical energy of these oxidants.
Fuel cells without the oxygen cathode are a promising approach to resolve many issues with oxygen reduction reaction. One of these approaches is to use redox flow batteries where another electrode and oxidant are used instead of Pt/oxygen. These batteries are based on reversible redox reactions and initially were suggested in NASA. They can be used in submarine applications to accumulate, to store and to use electric energy when necessary, but have the major challenge, which is nonselective ionic and water migration through the ion-exchange membrane.
A fundamentally different principle of voltage generation can be found in Nature. For example, an electric organ of an eel Electrophorus electricus is a stack of biological membranes. This organ during excitation can generate voltages up to 1000V and currents up to 1 A. There are no electrochemical reactions on electrodes, and because of the high packing density of the membranes it could be possible to make an artificial membrane-based source of electric energy with high power and small volume. The problem is that these biological membranes are not stable and cannot be used as an electrochemical power source in everyday life.
If an artificial ion-selective membrane separates two solutions with different activity of penetrating ions, this by itself should lead to the charge separation and generation of transmembrane electric potential. If the membrane has high permeability of these ions and, as the result, low electric resistance, this also could be used as an electrochemical cell. These electrochemical cells are called concentration elements, but they did not find practical applications because of the low generated voltage. For an ideal ion-selective membrane, according to the Nernst Law 10 times concentration difference at room temperature leads to less than 60 mV voltage. Even if pH difference is 14, an ideal voltage is only 840 mV, which is much less than the voltage, which is possible to generate using redox reactions. This voltage is generated on a pH-selective glass membranes in pH electrodes, but extremely high membrane resistance does not allow using it as a source of electric energy. To make a good battery it is not enough to have high voltage. In addition, the internal resistance of the battery including the membrane must be low to have reasonably high electric current.